The present invention relates to a dynamic viscoelasticity measuring apparatus.
Conventionally, in such apparatus both an amplitude "Xo" and a phase delay time ".tau." of sinusoidal strain with respect to sinusoidal stress are measured. The amplitude and phase delay time are needed to calculate basic physical quantities for a viscoelasticity measurement, i.e. loss tangent tan .delta., loss elastic modulus E", and storage elastic modulus E'. T and Xo can be measured in accordance with the following methods, which will be described with reference to FIG. 3:
1) T-measuring method: There are provided a force generator for applying a stress having a sinusoidal variation with time to a sample and a zero-crossing detector in a strain detector for detecting a corresponding sinusoidally varying strain of the sample. It is assumed that time measured after a zero crossing point of a sinusoidal stress waveform, or signal, has been detected and until a zero crossing point of sinusoidal strain is detected, is equal to .tau..
2) Xo-measuring method: There is employed a maximum value, or upper-side peak, detecting circuit for a sinusoidal strain waveform, or signal, and a minimum value, or lower-side peak, detecting circuit for this sinusoidal strain waveform. Half the difference between the upper-side peak "Xa" of the strain waveform and the lower-side peak "Xb" is regarded as an amplitude Xo of the sinusoidal strain waveform.
In the above-described conventional measuring methods, if high-frequency noise is present in the sinusoidal strain waveform, then the sinusoidal strain waveform could have a zero point crossing earlier than would a noise-free sinusoidal strain waveform due to the high-frequency noise in the phase difference detection. In the case of amplitude detection, high-frequency noise is likely to result in detection of a higher maximum amplitude value and/or a lower minimum amplitude value of the sinusoidal strain waveform than would be detected if such noise were not present in the strain waveform. It will be noted that the only parts of the strain waveform signal used to obtain measurement data are the zero crossing points and the maximum and minimum peaks of the strain waveform and as long as the stress signal is sinusoidal a noise-free strain signal will be at least substantially sinusoidal.
The above-described errors caused by noise may be eliminated or mitigated by employing a means for Fourier-transforming the strain waveform to derive a strain waveform having the same frequency as the stress waveform. As a consequence, precision of measurement data may be improved, errors due to noise may be lowered, and the useful measurement range for elastic modulus may be expanded.